Python parent class access to child private variables

Question

The following code generates an error:

class A(object):
    def say_something(self):
        print(self.foo)
        print(self.__bar)

class B(A):
    def __init__(self):
        self.foo = 'hello'
        self.__bar = 'world'

test = B()
test.say_something()

Printing of 'hello' is successful but 'world' generates the following error:

    print(self.__bar)

AttributeError: 'B' object has no attribute '_A__bar'

I am surprised by this, I would like my parent class to have a method which has access to a private attribute its children are guaranteed to have. Is there some standard way of solving this problem?

Answer 1

You can use self._B__something to access it. However, this is not what yuo should do. The proper solution is renaming __bar to _bar.

The idea behind the double-underscore name mangling is to avoid conflicts with subclasses/superclasses - it's not meant to be used for private variables. If a variable should be considered private/internal, prefix it with a single underscore. This does not cause any special treatment but every python developer will know that it's not part of the public API of the class/module containing that variable.

Answer 2

It's name mangling. Any member that starts with double underscores gets name mangled in Python. Your code would work with any other members, as long as they didn't start with double underscore.